I have a friend who is playing with me on a server we've created. Unfortunately, he is on a Mac whereas I am on a Windows machine. I told him that I should be able to compile and send a .minecraft folder to him so that we can both play. Unfortunately, he can't find his .minecraft folder where it is supposed to be. Is it hidden and how can I unhide it?

No, it's not hidden. It's just in an hard-to-find spot. The easiest way to find it is to just use the "Open texture pack folder" button from within Minecraft itself (located in Options → Texture Packs). On a Mac from within vanilla Minecraft, this will open ~/Library/Application Support/minecraft/texturepacks/, and then you simply have to go up one folder (⌘↑) to get to the minecraft folder at:

~/Library/Application Support/minecraft/

However, if you're using a different launcher for the client (for example, if your server is FTB or Technic, the client files are stored elsewhere so that vanilla Minecraft is left intact and unmodded), the minecraft folder will be in a different location, so you can't simply go directly to ~/Library/Application Support/ and expect to find it.

Using the texture pack button will always get you to the correct folder for whichever client you're using no matter whether it's modded or not, so it's the most reliable method.

I'd highly advise against showing all hidden files but you can run this chflags nohidden ~/Library/ in the Terminal and it will make your "Library" folder (for your user only) visible. You can then follow the directions indicated by other answers (e.g. navigating to Library -> Application Support -> Minecraft from your home directory).

PS: You can hide your Library folder again by running chflags hidden ~/Library... not that you'd want to do that though!
PPS: For the most part, I use ⌘+↑+G to quickly jump to locations like that. It even does auto-complete (start typing and hit tab). If that's the only "inaccessible" folder you navigate to, you'll be glad to know OS X keeps the last used command there so you can just hit ⌘+↑+G followed by Return and you're there. :)